Optimized discrete level sensing system for vehicle reductant reservoir

ABSTRACT

An example emissions-control system of a vehicle includes a reservoir configured to contain a reductant solution, and an SCR device disposed in the exhaust system and configured to consume the reductant solution. The example emissions-control system further includes a base sensor responsive to whether a volume of the reductant solution exceeds a base volume, wherein the base volume is a sum of a dead volume of the reservoir plus a standard volume, and one or more elevated sensors corresponding to one or more elevated volumes. The example emissions-control system further includes an emissions sensor responsive to a NOX level in the exhaust system, and a misformulation indicator operatively coupled to the emissions sensor and to at least one of the base sensor and an elevated sensor, and configured to indicate when an excess NOX emission follows, within an interval, an increase in the volume of the reductant solution above the base volume or an elevated volume. The example emissions-control system may further include an insufficiency indicator operatively coupled to the base volume sensor and configured to indicate when the volume of the reductant solution becomes less than the base volume.

TECHNICAL FIELD

The present application relates to the field of emissions control in vehicles, and more particularly, to control of nitrogen-oxide emissions from diesel-powered vehicles.

BACKGROUND AND SUMMARY

An emissions-control system in a vehicle may include a selective catalytic reduction (SCR device) device, wherein nitrogen oxide (NOX) emissions from an engine are combined with ammonia to form dinitrogen and water vapor. Ammonia may be supplied to the exhaust system of the vehicle via a reductant solution, e.g. aqueous urea. Maintenance of the quantity and quality of the reductant solution enables the benefits of the SCR device to be achieved through the operation of the system.

Therefore, U.S. Pat. No. 6,363,771 B1 describes a diagnostic system for a vehicle. The diagnostic system described therein is configured to detect when a level of the reductant solution in a reservoir descends below a threshold, and, based on such detection, to alert the operator of the vehicle when a refill is necessary. Further, the diagnostic system may be configured to detect when the solution contained in the reductant reservoir is misformulated, based on an adverse effect of misformulated reductant solution on NOX emissions. To determine whether NOX emissions are adversely affected, the diagnostic system interacts with the emissions-control system of the vehicle.

However, the inventors herein have recognized a flaw in this approach. Specifically, a diagnostic system as described above may not be able to distinguish a misformulated reductant solution from other causes of emissions-control error. If the diagnostic system determines incorrectly that the reductant solution in a vehicle is misformulated, it may issue erroneous warnings or take inappropriate corrective action—limiting speed or engine power, for example—which may be antagonistic to the operator of the vehicle. Therefore, the inventors herein have provided a way to test for insufficient or misformulated reductant in an integrated approach that heuristically distinguishes the misformulated reductant from other causes of emissions-control error.

In one embodiment, an example emissions control system of a vehicle is provided. The emissions-control system is operatively coupled to an exhaust system of the vehicle; it includes a reservoir configured to contain a reductant solution, and an SCR device disposed in the exhaust system and configured to consume the reductant solution. The example emissions-control system further includes a base sensor responsive to whether a volume of the reductant solution exceeds a base volume, wherein the base volume is a sum of a dead volume of the reservoir plus a standard volume, and, a series of elevated sensors corresponding to a series of elevated volumes, wherein each elevated volume is a product of the base volume times a positive integer power of two, and wherein each elevated sensor is responsive to whether the volume of the reductant solution exceeds a corresponding elevated volume. The example emissions-control system further includes an emissions sensor responsive to a NOX level in the exhaust system, and a misformulation indicator operatively coupled to the emissions sensor and to at least one of the base sensor and any elevated sensor, and configured to indicate when an excess NOX emission follows, within an interval, an increase in the volume of the reductant solution above the base volume or any elevated volume in the series of elevated volumes. The example emissions-control system may further include an insufficiency indicator operatively coupled to the base volume sensor and configured to indicate when the volume of the reductant solution becomes less than the base volume.

Another embodiment provides a method of evaluating a reductant solution stored on-board a vehicle in a reservoir, the vehicle having a reductant-delivery system for delivering the reductant solution to an SCR device in an exhaust system of the vehicle. This example method includes varying a reductant-solution delivery rate in response to a reductant-solution concentration to maintain emissions-control performance, and distinguishing misformulation of the reductant solution from other emissions-control system errors based on a volume change in the reservoir in combination with an emissions-control performance assay.

Still other embodiments provide different emissions-control systems and related methods to detect at least one of an insufficient reductant solution and a misformulated reductant solution in vehicle. In addition to numerous other advantages, these systems and methods may reduce the likelihood that an error sensed by an emissions-control system in a vehicle will be falsely attributed to improper maintenance of the reductant solution, thereby enabling more accurate diagnosis of the cause of the error.

It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows elements of an example emissions-control system of a vehicle, in accordance with the present disclosure.

FIG. 2 shows elements of an example emissions-control system of a vehicle equipped with an example level sensor, in accordance with the present disclosure.

FIG. 3 illustrates an example method to provide ammonia to an SCR device disposed in an exhaust system of a vehicle, in accordance with the present disclosure.

DETAILED DESCRIPTION

FIG. 1 shows elements of an emissions-control system of a vehicle in one, example embodiment. Emissions-control system 100 includes SCR device 102 and reservoir 104. The SCR device is disposed in an exhaust system of the vehicle; it includes a catalytically active surface, where nitrogen-based reducing agents (ammonia, urea, etc.) may react with NOX, forming dinitrogen and water vapor. The reservoir is configured to contain a liquid, which is intended to be a reductant solution appropriately matched to the SCR device. For example, the liquid may be an aqueous urea solution of a concentration recommended for use with the SCR device. Further the capacity of the reservoir may be chosen based on a nominal rate, u_(N), of delivery of the reductant solution to the SCR device, and further based on a desired mileage range of the vehicle. For example, the capacity of the reservoir may be sufficient to allow the vehicle to travel the desired mileage range, while consuming the reductant solution at the nominal rate of consumption.

Emissions-control system 100 includes reductant-delivery system 106, which is configured to draw liquid from reservoir 104 at the nominal rate u_(N). As shown in FIG. 1, the liquid drawn from the reservoir is delivered to SCR device 102. Reductant-delivery system 106 may include any pump suitable for delivering an aqueous solution from the reservoir to the SCR device; it may include a mechanical pump—a centrifugal pump, a reciprocating pump, etc.—or, in other embodiments, a source of compressed air or gas configured to pressurize the reservoir and force the liquid therefrom. In some embodiments, the reductant-delivery system may be configured to draw liquid from the reservoir at a plurality of different rates, which include the nominal rate u_(N), rates less than the nominal rate, and rates greater than the nominal rate. In one embodiment, the reductant-delivery system may be configured to draw liquid from the reservoir over a range of rates from zero (no delivery) to a maximum rate u_(M)>u_(N).

The volume of liquid contained in reservoir 104 may vary during operation of the vehicle. Thus, FIG. 1 shows a variable level L of liquid in the reservoir, below which level a variable volume V of liquid is contained. FIG. 1 also shows certain fixed volumes within the reservoir; these include dead volume D and base volume B. Dead volume D is a volume of liquid that cannot be reliably withdrawn from the reservoir by reductant-delivery system 106. The dead volume is a consequence of the detailed configuration of the reservoir and the reductant-delivery system. Although the reservoir and the reductant-delivery system may be configured to minimize the dead volume, various design considerations may result in some dead volume remaining.

Emissions-control system 100 includes base sensor 108, insufficiency indicator 110, and controller 112. The base sensor may be a sensor responsive to whether variable volume V exceeds base volume B. In one embodiment, the base sensor may an appropriately configured level sensor, as described hereinafter, with reference to FIG. 2. The insufficiency indicator may include various indicators—visual, audible, etc.—configured to indicate when variable volume V falls below base volume B. To that end, the insufficiency indicator may be operatively coupled to the base sensor via the controller. In some embodiments, activation of the insufficiency indicator may initiate a sequence of derating conditions in the vehicle intended to limit engine output when reductant solution for the SCR device becomes unavailable. The derating conditions may include, for example, vehicle speed governance, throttle governance, ignition suppression, etc. The emissions-control system may be configured to reset the insufficiency indicator when the variable volume V exceeds base volume B, as reported by the base sensor.

In some embodiments, base volume B is the sum of dead volume D plus a standard volume S. The standard volume S may be a fixed volume; it may be a unit volume or an integer multiple of a unit volume: one gallon, three liters, etc. In some embodiments, the standard volume may be the volume of reductant solution provided in a purchasable container of reductant solution appropriate for use in the vehicle. The standard volume may be selected in this manner to ensure that adding the entire contents of one purchasable container of reductant solution will result in variable volume V exceeding base volume B. As indicated above, this condition may trigger a reset of insufficiency indicator 110, and thereby suspend one or more derating conditions that may be in place.

Emissions-control system 100 further includes emissions sensor 114. The emissions sensor may be any sensor responsive to a NOX level in the exhaust system of the vehicle. The emissions sensor may be configured to perform an emissions-control assay, e.g. to detect, infer, or measure NOX level in the exhaust system, a level of any NOX constituent, parameters indicative of emission levels, or another condition correlated to exhaust NOX levels. The emissions sensor may further be configured to sense or detect an excess NOX level in the exhaust system. In some embodiments, reductant-delivery system 106 may be operatively coupled to emissions sensor 114 via controller 112, in a closed-loop manner. For example, the controller may be configured to vary the rate of delivery of the reductant solution to SCR device 102 so as to use as little reductant solution as possible while maintaining the NOX level below an acceptable limit. In doing so, the controller may vary the rate of delivery of the reductant solution among a plurality of delivery rates that the reductant-delivery system is configured to provide.

During a course of operation of the vehicle, liquid may be added to reservoir 104 according to various schedules or scenarios. Some scenarios include accidental or deliberate misformulation of the reductant solution by an operator of the vehicle. For example, some operators may delay adding reductant solution to the reservoir until insufficiency indicator 110 becomes active and indicates that a derating condition may follow. The operator of the vehicle, aware that the derating condition is linked to the volume of liquid in the reservoir, may add liquid at that time, thereby resetting the insufficiency indicator and suspending the derating condition.

Ideally, the liquid that the operator adds to reservoir 104 will be a reductant solution of a composition and concentration appropriate for the vehicle. But in some cases, an operator may add a different liquid, resulting in the reductant solution becoming misformulated; the operator may add reductant solution of an incorrect composition, a too-dilute reductant solution, or water, for example.

Emissions-control system 100 may be configured to tolerate dilution of an otherwise-correctly formulated reductant solution within certain limits. For example, controller 112 may be configured to increase the rate of delivery of reductant solution to SCR device 102 when the nominal rate of delivery fails to maintain the NOX level below an acceptable limit. In one embodiment, reductant-delivery system 106 may be configured to draw the liquid from the reservoir at a nominal rate u_(N) and at a maximum rate u_(M), greater than the nominal rate. The controller may be configured to switch the rate of delivery of reductant solution from the nominal rate to the maximum rate as needed to maintain the NOX level below the acceptable limit. In other embodiments, reductant-delivery system 106 may be configured to draw the liquid from the reservoir over a range of rates from substantially zero to the maximum rate u_(M). Such embodiments may admit of a threshold concentration of reductant solution below which the NOX level cannot be kept below the acceptable limit. For example, a threshold concentration C_(M) may be related to the nominal rate u_(N) and to the maximum rate u_(M) by C_(N)u_(N)=C_(M)u_(M), where C_(N) is a nominal concentration of reductant solution intended for use in the vehicle.

When the reductant solution is further diluted, such that the concentration falls below C_(M), an emissions-control error may result. For example, emissions sensor 114 may indicate that a NOX level is above an acceptable limit, although reductant-delivery system 106 is delivering liquid to the SCR device at the maximum rate u_(M). Therefore, emissions-control system 100 further includes misformulation indicator 116, which is operatively coupled to emissions sensor 114 via controller 112 and configured to indicate that the reductant solution in reservoir 104 is misformulated. However, as the NOX level may exceed the acceptable limit due to various other malfunctions unrelated to the reductant solution, emissions-control system 100 may be further configured to apply a heuristic to assess whether the emissions-control error is likely caused by a misformulation of the reductant solution or whether some other fault is indicated. In one example, the operation described herein enables such a heuristic using a straightforward and inexpensive configuration—one that comprises a few discrete level sensors, as opposed to more costly alternatives. Nevertheless, the approach described herein may be combined with such other approaches without departing from the scope of this disclosure.

The following illustrates one example heuristic for assessing a cause of emissions-control error according to the example configuration described herein.

An emissions-control error may occur when insufficiency indicator 110 is inactive, e.g., when variable volume V exceeds the base volume B. Let t_(B) be the time since the variable volume V last traversed the base volume, and let τ be the time required to substantially deplete and refill the ammonia atmosphere in SCR device 102 under current vehicle operating conditions. If t_(B)<τ, it may be considered likely that the emissions-control error is due to a misformulation of the reductant solution that occurred during the most recent addition of liquid to reservoir 104. However, if t_(B)>τ, it may be considered likely that the emissions-control error is unrelated to the most recent addition of liquid to the reservoir.

The above example shows how a time of traversal of the variable volume V above the base volume B may figure into a heuristic for assessing whether misformulation of the reductant solution is a likely cause of emissions-control error. It is possible, however, that misformulation of the reductant solution may occur and may cause an emissions-control error when the variable volume does not traverse the base volume. To enable assessment under such conditions, emissions-control system 100 further includes elevated sensor 118A, which may be a sensor responsive to whether the variable volume V exceeds elevated volume E₁. In one embodiment, the elevated sensor may an appropriately configured level sensor, as described hereinafter, with reference to FIG. 2. As shown in FIG. 1, elevated volume E₁ may be the product B×R, where R=u_(M)/u_(N).

Suppose that the variable volume V is between the base volume B and the elevated volume E₁, that the NOX level is below the acceptable limit, and that reductant-delivery system 106 is supplying the reductant solution at the nominal rate u_(N). It will be observed that dilution of the reductant solution to a volume less than E₁ will yield a concentration greater than the threshold concentration C_(M). It should be possible, under such conditions, to avoid an emissions-control error by delivering the reductant solution to SCR device 102 at an accelerated rate, as described above. However, if the reductant solution is diluted such that the final volume exceeds E₁, it is then possible that the concentration of the reductant solution may fall below C_(M), triggering an emissions-control error.

The example heuristic may now be extended as follows. An emissions-control error may occur, as before, when insufficiency indicator 110 is inactive. Let t_(E) be the time since the variable volume V last traversed elevated volume E₁. If t_(E)<τ, it may be considered likely that the emissions-control error is due to a misformulation of the reductant solution that occurred during the most recent addition of liquid to reservoir 104. However, if t_(E)>τ, it may be considered likely that the emissions-control error is unrelated to the most addition of liquid to the reservoir.

Thus, at a heuristic level, assessment of whether an emissions-control error is due to misformulation of the reductant solution may be based on whether at least one of a base volume and an elevated volume have been exceeded in an interval preceding the emissions-control error, wherein the interval corresponds to the time required to substantially empty and refill the ammonia atmosphere in SCR device 102. Therefore, in one embodiment, the misformulation indicator may be operatively coupled to emissions sensor 114, to base sensor 108 and to elevated sensor 118A. The misformulation indicator may be configured to indicate when an excess NOX emission follows, within the interval, an increase in variable volume V above at least one of base volume B and elevated volume E₁.

As illustrated in FIG. 1, elevated sensor 118A may be one in a series of elevated sensors (i.e., 118A, 118B, 118C, etc.) corresponding to a series of elevated volumes, wherein each elevated volume E_(i) is given by E_(i)=B×R^(i), and wherein each elevated sensor is responsive to whether the variable volume V exceeds a corresponding elevated volume E_(i). Further, in embodiments that comprise a series of elevated sensors, misformulation indicator 116 may be further configured to indicate when an excess NOX emission follows, within the interval described above, an increase in variable volume V above any elevated volume E_(i) in the series of elevated volumes.

The functionality of emissions-control system 100 is most easily understood with reference to a non-limiting, example embodiment in which the maximum delivery rate u_(M) is substantially twice that of the nominal delivery rate u_(N), i.e., R≈2. In this embodiment, the series of elevated volumes comprise a geometric series, E₁=2B, E₂=4B, E₃=8B, etc.

It will be understood that emissions-control systems fully consistent with this disclosure may include various other components not shown in FIG. 1: lean NOX traps, diesel oxidation catalyst modules, diesel particulate filters, as examples.

In some embodiments, one or more of base sensor 108 and elevated sensors 118A, 118B, etc. may include a level sensor. Therefore, FIG. 2 shows, in schematic detail, an example level sensor 200. As described hereinafter, a level sensor may be configured to sense whether a variable volume V of liquid in a reservoir traverses a fixed volume (e.g., the base volume or an elevated volume). It will be understood, however, that some embodiments fully consistent with this disclosure may employ an approach other than level sensing to sense whether a volume of liquid in a reservoir traverses a fixed volume.

Continuing in FIG. 2, level sensor 200 includes level-sensing element 202, which is held in place by support structure 204. The level-sensing element may be any element-electronic, optical, acoustic, etc.—responsive to whether a variable level L of liquid approaches a threshold level L₀ to within a tolerance interval, which may be any interval that brackets the threshold level L₀. For example, the tolerance interval may be the interval L₀−α≦L≦L₀+β, where the parameters α and β may or may not be equal (vide infra).

As shown in FIG. 2, level-sensing element 202 is operatively coupled to controller 112; thus, the controller may be configured to provide a bias to the level-sensing element as well as register a response therefrom. In some embodiments, the level-sensing element may give a steady response whenever the variable level is within the tolerance interval, while in other embodiments, the level-sensing element may respond transiently to variable level L passing into or out of the tolerance interval. Further, the level-sensing element may be configured to respond differently depending on whether the variable level passes into the tolerance interval from above or whether it passes into the tolerance interval from below. Such functionality may be due to a configuration of the level sensing element, the controller, or both.

In the illustrated embodiment, level-sensing element 202 may respond to variable level L approaching threshold level L₀ because it is disposed substantially at the threshold level, is immersed in liquid when the variable level is above the tolerance interval, and is not immersed in liquid when the variable level is below the tolerance interval. In such embodiments, support structure 204 fixes the vertical position of the level-sensing element and thereby fixes the threshold level L₀ relative to the lowest point in reservoir 104. In FIG. 2, the support structure couples the level sensing element to an inside surface of the reservoir. It will be understood, however, that various other embodiments are contemplated, including those in which the support structure extends downward from the top of the reservoir, thereby suspending the level sensing element a fixed vertical distance from the top of the reservoir. In still other embodiments, the level sensing element may be responsive to the liquid level even if it is not immersed in the liquid (e.g., if it is disposed outside the reservoir).

In embodiments that employ a level sensor, such as level sensor 200, to determine whether a variable volume exceeds a fixed volume, the fixed volume V₀ and the threshold level L₀ may be related according to

V₀ = ∫_(h = 0)^(L₀)S(h) h,

where S(h) is a surface area of the liquid in the reservoir when a surface of the liquid is a height h above a lowest point inside the reservoir.

In one particular embodiment, wherein base sensor 108 includes level sensor 200, the fixed volume V₀ in the equation above may correspond to base volume B, and the tolerance interval may be disposed so that 0≦β<α. An asymmetric tolerance interval such as this may chosen so that a response of the base sensor to an addition of the standard volume of liquid to the reservoir, when the volume of liquid in the reservoir is initially below the base volume, occurs statistically at a 3σ level, based on expected operating conditions of the vehicle.

FIG. 3 illustrates an example method to detect at least one of an insufficient reductant solution and a misformulated reductant solution in a vehicle, in a manner consistent with the example configurations set forth above.

Method 300 begins at 302, where a reductant solution is contained within a reservoir. As noted above, the reservoir may be equipped with a level-sensing system. The level-sensing system may include a base sensor responsive to whether a volume of liquid in the reservoir exceeds a base volume B, where the base volume is a sum of a dead volume of the reservoir plus a standard volume. The level-sensing system may further include a series of elevated sensors corresponding to a series of elevated volumes E_(i), wherein each elevated volume is the base volume times a positive integer power of two, and wherein each elevated sensor is responsive to whether the volume of liquid in the reservoir exceeds a corresponding elevated volume.

Method 300 continues to 304, where it is determined whether a variable volume V of liquid in the reservoir exceeds the base volume B. If the variable volume does not exceed the base volume, then, at 306, insufficiency of the reductant solution is indicated. Insufficiency of the reductant solution may be indicated via an insufficiency indicator, substantially as described above. The indication of reductant-solution insufficiency may trigger, at 308, a warning chain; it may further trigger a derating sequence intended to limit emissions from the vehicle. It will be understood that ‘warning chain,’ as used herein, may comprise any inducement or series of inducements intended to encourage the operator of the vehicle to maintain the quantity and/or quality of the reductant solution. The warning chain may be responsive at least partly to a distance that the vehicle travels after reductant-solution insufficiency is indicated, e.g., from an output of a vehicle odometer or other distance-responsive vehicle component. In other embodiments fully consistent with this disclosure, the warning chain may be responsive at least partly to a number of engine revolutions or other suitable surrogate. In one particular embodiment, the warning chain may be based on an average rate of consumption of reductant solution (in milliliters per mile, for example) and on the estimated volume of reductant solution remaining in the reservoir, based on distance travelled after reductant-solution insufficiency is indicated. Thus, the average rate of consumption may be used to calculate various threshold distances used in the warning chain.

Continuing in method 300, if it is determined at 304 that the variable volume exceeds the base volume, then, at 310, any pre-existing insufficiency indication is cleared, and any associated warning chain and/or derating sequence is suspended. This step may further include indicating that reductant solution has been added to the reservoir.

Method 300 then continues to 312, where it is determined whether a NOX level in the exhaust system is below an acceptable limit. This determination may be pursuant to an emissions-control assay, which may be enabled by an emissions sensor disposed in an exhaust system of the vehicle, as described hereinabove. In some embodiments, this determination may include assessing whether the NOX level does or does not respond expectedly to an increasing rate of delivery of reductant solution to an SCR device of the vehicle. If the NOX level is below the acceptable limit, then execution resumes at 304. However, if it is determined that the NOX level exceeds the acceptable limit, then, at 314, it is determined whether the excess NOX emission has followed, within an interval, an increase in the volume of liquid in the reservoir above either the base volume or any elevated volume in the series of elevated volumes. In one embodiment, the interval selected for this purpose may include a time to deplete and refill a nominal operating amount of ammonia in the SCR device of the vehicle. If it is determined that the excess NOX emission has followed such an increase in volume within the interval, then, at 316, misformulation of the reductant solution is indicated. Misformulation of the reductant solution may be indicated via a misformulation indicator, substantially as described hereinabove.

However, if it is determined at 314 that the excess NOX emission did not follow, within the interval, an increase in the volume of liquid in the reservoir above either the base volume or any elevated volume in the series of elevated volumes, then, at 320, other warnings and/or diagnostics may be applied to assess the cause of the excess NOX emission. Then, from 316 or 320, the method returns to 304.

It will be understood that the example control and estimation routines disclosed herein may be used with various system configurations. These routines may represent one or more different processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, the disclosed process steps (operations, functions, and/or acts) may represent code to be programmed into computer readable storage medium in a control system. It will be understood that some of the process steps described and/or illustrated herein may in some embodiments be omitted without departing from the scope of this disclosure. Likewise, the indicated sequence of the process steps may not always be required to achieve the intended results, but is provided for ease of illustration and description. One or more of the illustrated actions, functions, or operations may be performed repeatedly, depending on the particular strategy being used.

Finally, it will be understood that the systems and methods described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are contemplated. Accordingly, the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and methods disclosed herein, as well as any and all equivalents thereof. 

1. A method of evaluating a reductant solution stored on-board a vehicle in a reservoir, the vehicle having a reductant-delivery system for delivering the reductant solution to an SCR device in an exhaust system of the vehicle, the method comprising: varying a reductant-solution delivery rate in response to a reductant-solution concentration to maintain emissions-control performance; distinguishing misformulation of the reductant solution from other emissions-control system errors based on a volume change in the reservoir in combination with an emissions-control performance assay.
 2. The method of claim 1, wherein the reductant-solution delivery rate is varied among a plurality of rates, which include a nominal rate and a maximum rate greater than the nominal rate.
 3. The method of claim 2, wherein misformulation of the reductant solution is indicated when an excess NOX emission occurs within an interval following the volume change, the interval including a time to deplete and refill a nominal operating amount of ammonia in the SCR device.
 4. The method of claim 2, wherein the volume change comprises an increase in the volume of liquid in the reservoir above at least one of a base volume and an elevated volume, where the base volume is greater than a dead volume of the reservoir, and the elevated volume is a product of the base volume times a positive-integer exponentiated ratio of the maximum rate to the nominal rate.
 5. The method of claim 2, wherein the base volume is a sum of the dead volume of the reservoir plus a standard volume, and the standard volume is a volume of reductant solution in a purchasable container of reductant solution appropriately matched to the SCR device.
 6. The method of claim 2, wherein maximum rate is substantially twice the nominal rate.
 7. An emissions-control system operatively coupled to an exhaust system of a vehicle, the emissions-control system comprising: a reservoir configured to contain a liquid; an SCR device disposed in the exhaust system; a reductant-delivery system configured to draw the liquid from the reservoir at a nominal rate and at a maximum rate, greater than the nominal rate, and further configured to pump the liquid to the SCR device; a base sensor responsive to whether a volume of liquid in the reservoir exceeds a base volume greater than a dead volume of the reservoir; an elevated sensor responsive to whether the volume of liquid in the reservoir exceeds an elevated volume, where the elevated volume is a product of the base volume times a ratio of the maximum rate to the nominal rate; an emissions sensor responsive to a NOX level in the exhaust-system; and a misformulation indicator operatively coupled to the emissions sensor and to at least one of the base sensor and the elevated sensor, and configured to indicate when an excess NOX emission follows, within an interval, an increase in the volume of liquid in the reservoir above at least one of the base volume and the elevated volume.
 8. The emissions-control system of claim 7, wherein the liquid comprises a reductant solution appropriately matched to the SCR device.
 9. The emissions-control system of claim 7, further comprising an insufficiency indicator operatively coupled to the base sensor and configured to indicate when the volume of liquid in the reservoir falls below the base volume.
 10. The emissions-control system of claim 7, where the elevated sensor is one in a series of elevated sensors corresponding to a series of elevated volumes, wherein each elevated volume E_(i) is given by E_(i)=B×R^(i), where B is the base volume, R is a ratio of the maximum rate to the nominal rate, i is an integer greater than zero, where each elevated sensor is responsive to whether the volume of liquid in the reservoir exceeds a corresponding elevated volume; and where the misformulation indicator is further configured to indicate when an excess NOX emission follows, within the interval, an increase in the volume of liquid in the reservoir above any elevated volume in the series of elevated volumes.
 11. The emissions-control system of claim 7, wherein the base sensor is responsive to whether a level of liquid in the reservoir approaches a threshold level to within a tolerance interval, where the base volume B is related to the threshold level Lo according to B = ∫_(h = 0)^(L₀)S(h) h, where S(h) is a surface area of the liquid in the reservoir when a surface of the liquid is a height h above a lowest point inside the reservoir.
 12. The emissions-control system of claim 11, wherein the tolerance interval is not symmetric about the threshold level.
 13. The emissions-control system of claim 11, wherein the tolerance interval is chosen such that a response of the base sensor to an addition of the standard volume of liquid to the reservoir, when the volume of liquid in the reservoir is initially below the base volume, occurs statistically at a 3σ level, based on expected operating conditions of the vehicle.
 14. The emissions-control system of claim 7, wherein the elevated sensor is responsive to whether a level of liquid in the reservoir approaches a threshold level to within a tolerance interval, where the elevated volume E_(i) is related to the threshold level L_(i) according to E_(i) = ∫_(h = 0)^(L_(i))S(h) h, where S(h) is a surface area of the liquid in the reservoir when a surface of the liquid is a height h above a lowest point inside the reservoir.
 15. A method to detect at least one of an insufficient reductant solution and a misformulated reductant solution in vehicle, the method comprising: containing the reductant solution in a reservoir equipped with a level-sensing system, comprising: a base sensor responsive to whether a volume of liquid in the reservoir exceeds a base volume, where the base volume is a sum of a dead volume of the reservoir plus a standard volume, and a series of elevated sensors corresponding to a series of elevated volumes, wherein each elevated volume is a product of the base volume times a positive integer power of two, and wherein each elevated sensor is responsive to whether the volume of liquid in the reservoir exceeds a corresponding elevated volume; indicating that the reductant solution is insufficient when the volume of liquid in the reservoir becomes less than the base volume; and indicating that the reductant solution is misformulated when an excess NOX emission follows, within an interval, an increase in the volume of liquid in the reservoir above any elevated volume in the series of elevated volumes.
 16. The method of claim 15, wherein the excess NOX emission includes a NOX level failing to respond expectedly to an increasing rate of withdrawal of reductant solution from the reservoir.
 17. The method of claim 15, further comprising initiating a warning chain when the volume of liquid in the reservoir becomes less than the base volume.
 18. The method of claim 15, further comprising applying other diagnostics when the excess NOX emission is detected, but does not follow, within the interval, an increase in the volume of liquid in the reservoir above any elevated volume in the series of elevated volumes.
 19. The method of claim 15, further indicating that reductant solution has been added to the reservoir when the volume of liquid in the reservoir exceeds the base volume.
 20. The method of claim 19, further comprising suspending the warning chain when the volume of liquid in the reservoir exceeds the base volume. 